The key objective of this study is to collect the first nationally representative data on prevalence and correlates of DSM-IV MDD from the recently completed National Comorbidity Survey Replication (NCS-R).The study design is a direct interview household survey of a probability household survey of adults ages 18 and over from the 48 contiguous United States. The total number of subjects in the study is 9090, representing a 79% response rate. The diagnostic interview was the WHO Composite International Diagnostic Interview (CIDI), developed to collect diagnostic criteria for the DSM-IV. Clinical re-interviews were carried out with the Structured Clinical Interview for DSM-IV (SCID) to validate CIDI diagnoses. The initial results of this study were published in July, 2003. We found that CIDI MDD prevalence estimates are 16.2% lifetime (32.6-35.1 million U.S. adults) and 6.6% 12-month (13.1-14.2 million U.S. adults). Sociodemographic correlates are female, younger than 60, unmarried, not black, unemployed, and disabled. Nearly three-fourths of MDD cases have comorbid anxiety, substance, or impulse control disorders. MDD is temporally primary in only a minority of these cases. Respondents with serious-severe 12-month MDD (38% of all 12-month MDD) averaged 83.5 days out of role, while mild-moderate cases averaged 9.6 days. Although a high proportion (72.4%) of serious-severe 12-month cases receive treatment, only a minority (36.9%) of treatment is adequate. Treatment adequacy is related to sector, but not severity. Sociodemographic correlates of severity, treatment, and treatment adequacy are far less numerous than the correlates of prevalence. In summary, we found that MDD is a common, seriously impairing, disorder that is widely distributed in the population. Early treatment of temporally primary disorders might be useful to prevent MDD. While the recent dramatic increase in treatment is encouraging, low treatment adequacy is a source of concern. The emphasis on screening, detection, and expansion of the number of MDD cases in treatment needs to be accompanied by a parallel emphasis on quality improvement.